The overall objective of this study is to gain an understanding of the developmental factors leading to the complex set of connections between neurons in the visual cortex that determine its function. Since the rules governing the development of the nervous system are genetically determined, an approach that holds considerable promise is one in which the experimental perturbation is genetic. The mammalian species best suited to the genetic approach is the mouse, Mus musculus. The reeler mutation (rl) produces a stereotyped abnormality in cell position in many cortical structures of the mouse, including the visual cortex. The abnormality of reeler cortex serves as an experimental lesion that poses a number of interesting questions concerning subsequent steps in cortical development. Therefore a major objective of this study will be to use the reeler mutant mouse to determine the role played by cell position in the development of afferent, intrinsic, and efferent connections of the visual cortex. The studies will be carried out using the techniques of microelectrode recording for the characterization of receptive fields of cortical cells, and neuronal transport of tracer substances to define neuroanatomical pathways. Our recently completed analysis of the laminar distribution of receptive field types in normal mouse striate cortex and the retinotopic organization of striate and extrastriate visual cortical regions will serve as the basis for the proposed studies. In view of the potential importance of mouse mutants for the study of the development of the visual cortex, a further objective will be to obtain additional information on the normal mouse visual cortex, in order to provide a framework for the analysis of other mutations that alter visual system development.